Ao

Was most probably born when Satomi Rio saved Sousuke's life five years ago, but at the cost of splitting his soul in half.

Ao also holds their childhood memories, which is the reason why the other Sousuke can't remember much.

Ao seems obsessed with Shino, he likes teasing Shino but maybe that is only a facade to hide his loneliness, because Shino chose to be with his other half, Sousuke.